Electroluminescent storage device



June 21, 1960 B. KAZAN 2,942,120

ELECTROLUMINESCENT STORAGE DEVICE /6 fLEcr/m- 5 MM/fvfscf//f /4 Mmm/4L a INVENTOR.

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ELECTROLUMINESCENT STORAGE DEVICE Filed Dec. l2, 1955 I5 Sheets-Sheet 3 ak ""fa@ Xu fsgf@ INVENTUR. Bye'g/amm jj; Mm

United States Patent O 2,942,120 4 ELEcrRoLUMmEscENr sroRAGE DEVICE Benjamin lazan, Princeton, NJ., assgnor to Radio Corporation of America, a corporation of Delaware Filed Dec. 12, 1955, Ser.`No. 552,389

6 Claims. (Cl. Z50- 213) This inventionrelates to electroluminescent devices and particularly to such devices designed to intensify and store light or light images. s

Itis known that luminescence can be produced in some phosphor materials by subjecting them to an electric field or current. The intensity of the luminescence increases with increasing electric field strength. These phosphor materials are known as electroluminescent materials or electroluminescent phosphors, and the property which they exhibit is known as electroluminescence. Some examples of electroluminescent materials are zinc sulfide activatedby` copper and-zinc selenide activated `by manga- Dese.

It is also known that certain materials possessthe property of being able to vary their electrical impedance in response to incident radiant energy, the conductivity increasing with b increasing radiation. ployed may be visible light, infra-red radiation, ultraviolet radiation, gamma rays, or X-rays, for examples. When the materials are responsive to light, they are known as photoconductive materials, and the property they exhibit is known as photoconductivity. Some examples of very sensitive photoconductive materials are cadmium `sulfide and cadmium selenide in crystalline form.

4It has `been proposed to intensifylight images by means of a projection screen formed of a layer of photoconductive material and a contiguous layer `of electroluminescent material sandwiched between two transparent sheet electrodes to which an alternating current voltage is applied. The relative thicknesses of the two layers are chosen for the materials used so that in the dark the l,impedance of the photoconductive layer is substantially higher than that of the electroluminescent layer. Under these conditions, fa. greater fraction of the supply voltage will be impressed across the photoconductive layer in the dark than across the electroluminescent layer. The supply voltage is adjusted so that in the dark the voltage appearing across the electroluminescent layer is below the threshold voltage, thatis, below the voltage required to cause visible luminescence.

i When a light image is projected onto the surface of the photoconductive layer, the conductivity of the layer is in- .creased in elemental areas in amounts corresponding to the intensity ofthe light which strikes. The impedance of the photoconductive layer inthe excited areas drops accordingly, resulting in a decrease in the amount of voltage appearing `across the photoconductive layer; a corresponding increase in the electric eld impressed `across the elemental areas of the electroluminescent layer is` produced. Light is thus emitted from elemental areas lof the electroluminescent layer which varies in intensity with` the strength of the electric field thereacross, so as `to produce an amplified light 'image corresponding to the incoming image.

Such a device intrinsically is capable of operating with light feedback. That is, the photoconductive layer can see some of the light emitted by the electroluminescent The radiation emf 2,942,120 Patented June 21, 1960 ICC layer, and hence can be excited by this light in a manner similar to the incident light. However, theiuse of such a device in the form described for the purpose of storing light or light images has certain limitations. In the first place, the electroluminescent layer plays a dual role of providing the lightv for viewing as well as for feedback. Since -the feedback light must match the `spectral response of the photoconductive material, `this light `may ,not be of a desirable color for viewing, for exciting Vanother device, or for being photographed. Anothertlimitation, which aifects the feedback efficiency, is that the photoconductive layer, which must be made substantially thicker than the electroluminescent layer to provide a desired impedance ratio for adequate control of the electroluminescent light, can not be excited to its full depth by the electroluminescent light. Still another limitation is the fact that ambientlight on the phosphor side of the device must be maintained at a very low 'level to prevent this light from passing through `the phosphor layei and exciting the photoconductor. Still another disadvantage results from the effects of cross-modulation. That is, some of the electroluminescent light emitted from a given area may be reilected back to or directly excite an adjacent area of the photoconductive layer which normally should be dark. The result is a triggering on of dark areas and a progressive spreading of the electroluminescent lighttd the extent that light is emitted from the entire surface areaand all picture detail is lost.

Accordingly, it is an object of this invention to provide `an electroluminescent storage device which has a high ldegree of storage efliciency.

A further object is to provide an electroluminescent image device in which storage action can occur Iwith a minimum of cross-modulation and with full detail.

A further object is to provide an electroluminescent storage device which can be viewed in high levels of ambient light.

A further object is to `provide an electroluminescent image device which can amplify and retain images which are weak or of short duration.

Yet another object is to provide an electroluminescent image storage device .in which the output or viewed light may be of a different color than the` input and feedback light.

The foregoing and other objects are achieved in accordance with the invention by providing an electroluminescent storage device having two optically separate sections, namely, a storage section and an indicatingsection electrically in series. The `storage section includes one or `more-photoconductive elements and an electroluminescent element in series and arranged in light feedback relationship. The indicating section includes another electroluminescent element responsive to the variations in current ow in the storage section. Means are provided for shielding the photoconductive element in the storage section from the light emitted from the electroluminescent element of the indicating section and from ambient light.

The present invention is generally useful for the purpose of storing picture information or a light pattern. It has particular utility for recording light images of short duration, such as a single frame of a television picture, the recording of a transient oscilloscope trace, or the storage of radar information, for example.

`In the drawing:

Fig. l is a diagrammatic view partly in section of one elemental storage device according to the invent-ion;

Fig. 2 is a diagrammatic view partly in section of another elemental storage device according to the invention. i

Fig. 3` `is a broken fragmentary plan view with portions broken away of a multi-element storage `device insac-4 cordance with the invention;

Fig;` 4 riszalsectironalview' taken along the line 4-4 of Fig. 3;Y l

iFig. 5 is a partial sectional View of another form of multi-element storage devicel according to the invention; and `Y `f ff'Fig. 6 isv a' sectional view taken along the line 6 6 of Figui., V f I Y 1 g, Fig. 1 shows fan elemental storage device 10 which comprises a base I12 and three superposed layers on said base including YVa. frstphotoconductive layer 14, an electroluminescent phosphor layer 16, and a second photocon- 'ductive'flayer 18, in that o rder, the three layers being sandwiched between two-conductive layers 20 and' 21, which serve as electrodes. g Y Y The photoconductive layers 14 and 18 may be made Aof any of the well known materials having a variable impedance'characteristic in response to radiant energy. Examples of photoconductive materials are cadmium suliide and cadmium selenide which may be mixed with suitable dielectric binders.V Cadmium sulfide and cadmium selenide crystals in powder form are disclosed in copending application of Charles J. Busanovich and Soren M. Thomsen, Serial No. 472,354, iiled December l, 1954, now U.S. Patent 2,876,202. f A

The electroluminescent layer 16 may comprise any of the well known electroluminescent phosphors, preferably embedded in a transparent dielectric binder. Zinc sulfide activated by copper and zinc selenide activated by manganese are examples of suitable 'phosphors Eethyl cellulose and polystyrene are examples of binders.

The' conductive layers 20 and 21 may be made of gold, silver, or aluminum, and may be applied by evaporation, painting or spraying for example. At least one of the conductive layers 20 and 21, ifmade of metaLis applied thin enough to render it transparent to light. The base .12 may be made of transparent material, for example glass, if the device is tooperate by incident light passing through the base 12. If the base 12 is made of glass, the conductive coating 20 in contact therewith may be made of tin oxide or of transparent metal.

In operation, a source voltage 22, is connected across theelectrodes 20 and 21 vthrough one pole 23 of aswitch 24. The source may be alternating current of the order of 600 volts and having a` frequency of about 1000 cycles per second,depending :on the thicknesses of the photoconductive `and electroluminescent layers. The lforegoing values Vof voltages are suitable for operating a device in which the thickness of each photoconductive 'layer is about 5 mils and the thickness of the" electroluminescent layer is about 1 mil.' I

The device 10 may be shielded from ambient light by placing it in an' enclosure 25 made of opaque material and large enough to accommodate the device 10 and the light source with which the device 10 is designedto be operated. Y Y

In the absence of light on the device10, the impedance of each photoconductive layer is very high compared to the impedance of the electrolurninescent'layer. Il "herefore, a greaterV proportion of the applied voltage appears across each `photoconductive layer. than across thepelectroluminescent layer. The supply voltage may be adjusted so that the electroluminescent layer is operating below its threshold voltage, that is below the voltage required to just barely produce visible electroluminescence.

VIf an amount of light L1 is caused to irradiate the second photoconductive layer 18, the light causes the impedance of the photoconductive layer 1S to reduce. This'reduct-ion in impedance causes increased current ow through the ,series circuit, and the increased current flowing through'the electroluminescent layer 16 causes it to emit light. :The light emission from Vthe electroluminescent layer 16 excites both photoconductive. layers, causing a reduction in the impedance of the rst photoconductive layer 14i11 addition to sustaining the reduced I impedance in the second photoconductive layer18..V Since all or substantially all of the electroluminescent light is trapped by the photoconductive layers, the device 10 is an eflicient regenerative device. When the incident light L1 is removed, the regenerative action produced by light feedback of the electroluminescent light to the photoconductive layers maintainsthe photoconductive layers at a level of increased conductivity, thus sustaining e1ectroluminescence and storing the emitted light.

In order to render the storage action easily visible, the storage device 10 Vmay be coupled to an electroluminescent element or cell 26. For this purpose, the arm of switch 24 is connected toV the other pole 27 to place the electroluminescent cell 26 in series`with the storage device 10 and theV voltage sourceV 22.I In the circuit arrangement shown, the storage device 10 functions as a storage section, and the electroluminescent cell 26 functions as an indicating or viewing section. ,The cell 26 is constituted of an electroluminescent phosphor layer 28 sandwiched between two sheetelectrodes 30 and 32, at least one of which is transparent to light. The phosphor layer 28 may be about 1 mil thick and may have an area substantially equal'to the area of phosphor layer 16 between the electrodes 20 and 21.

With the dimensions of the storage ldevice 10 and elec- Vtroluminescent cell 26 being so chosen, the impedance of the storage device 10 in the unexcitedr'condition will be substantially higher than that of the"electroluminescent cell 26.v Insufiicientcurrent flows in theV series circuit to causeveitherof the phosphor layers 16 0128 to electroluminescence. When light L1 is caused to excite the storage-device 10, however, the resulting increased current ilows through both phosphor layers and ythey both emit light. The light emitted by the phosphor layer 16 of the storage device feeds back to the photoconductive vlayers 14 and 18 to keep the device l'regen'erating after the incident light L1 has been removed. The light Lg emitted by the phosphor layer28of the electroluminescent cell is visible through the transparent electrode, e.g. 32, and thus gives an indication'of the storage0 action of the device 10. Y h h Y f The device of Fig. 1 has improved feedback efficiency as compared with a device in which a single photoconductive layer and a single electroluminescent layer is used, inasmuch as all the" electroluminescent light is trapped by the -two photoconductive layers, each of which is only one-half the thickness of the single photoconductive layer. However, due to the fact that the current ows in `the direction of the thickness of each photoconductive layer of the device of Fig. l, the thickness required of each photoconductive layer, even though reduced by onehalf, must be large as compared with the electroluminescent layer.

The sensitivity may be improved .stillfurther by utilizing very thin photoconductive layers 14a and 18a, on opposite sides ofan electroluminescent layer 16, in an electroluminescent device 10a as shown in Fig.. 2. Two electrodes 20a and 21a, in the form of narrow strips or lines of conductive material, are supported on the opposite surfacesv of the electroluminescent `layerv16 and are spaced laterally from each other by a distance which is substantially longl as compared withthe thickness of the electroluminescent layer. One of the electrodes 20a contacts the first photoconductive'layer 14a and the other electrode 21a contacts the second photoconductive layer 18a. The materials used for the electroluminescent layer 16, the photoconductive layers 14a and 18a and the electrodes 20a and 21a may be the same as described in connection with Fig. l. The thickns of each of the-photoconductive layers 14a and 18a may be the same as or' less than the thickness of the electroluminescent layer 16. With a source 22 of 600 volts alternating current, as above described, the layers 14a, 18a and 16 may each be about 1 mil in thickness,`and the 'electrode spacing may be about 20 mils,

In the absence of light on the device 10a, the impedance between the electrodes 20a 'and 21a is very high along any path. Because of this high impedance, the electric current is very low and primarily capacitive and, although it must necessarily iiow through the electroluminescent layer 16, this current will be insufficient to cause visible electroluminescence of the layer 16.

If an amount of light L1 is caused `to irradiate the second photoconductive layer 18a, the light, if sutliciently bright, will penetrate all three layers, they being all rather thin. Both photoconductive layers 14a and `18a thus become more conducting and allow increased current to flow through the electroluminescent layer 16 along a multiplicity of current paths of substantially equal impedance as indicated by the arrows (i) in Fig. 2. In practice, a weak light may be used, illuminating only one photoconductive layer. This will canse electroluminescent light Vto be emitted initially nearest the electrode on the other photoconductive layer. This light, however, Will rapidly spread until the entire electroluminescent element emits light and both photoconductive layers are illuminated.

When the device a is excited, the electroluminescent layer 16 is caused to electroluminesce, by virtue of the increased current flow, and emit light from both surfaces thereof. The emitted light is of higher intensity than the incident light, the added power lto produce ampliiication lbeing supplied by the voltage source 22. Since the emitted light continues to illuminate both photoconductivelayers, further increase in conductivity of the photo conductive layers is produced, causing further increased electroluminescent current iiow and more light. The build-up inlight emission continues until an equilibrium determined by the supply voltage is reached. When the incident light L1 is removed, the regenerative action produced by light feedback of the electroluminescent light to the photoconductive layers maintains the photo conductive layers at a level of increased :conductivity thus sustaining electroluminescence and storing the emitted light.

The electroluminescent light of the storage device 10a is largely confined within the layered structure itself, due to the absorption of the light by the photoconductive layers 14a and 18a which practically enclose the electroluminescent layer 16. The feedback efficiency is very high because all the available electroluminescent light is utilized for the feedback process. The required long photoconductive path necessary for controlling an electroluminescent element is satisfied by the lateral spacing of the electrodes. 'Ihe use of a thin layered structure per-` mits incident light to efiiciently illuminate the photoconductor and coupled With a high feedback efficiency allows one to store a transient light signal which is relatively dim orof short duration.

An electroluminescent cell '26 may be coupled to the device 10a through a switch 24, in order to indicate the operation of the device 10a, as in Fig. l. Also, a light opaque enclosure 25 may be provided to optically shield the device 10a from ambient light'and from the electroluminescent light of the cell 26. The electrodes 20a and 21a may be located on the outer surfac of ythe photoconductive layers 14a and`18a, like the conductive layers 20 and 21 of Fig. l. In this case, the current path between electrodes 20a and 21a would be slightly longer by the amount of the thicknesses of the two photoconductive layer 14a and 18a.

Figs. 3 and 4 show a broad area device incorporating the features described above and designed to amplify and store light images. This device includ a transparent insulating base 34, such as a glass plate, which supports on a surface thereof a multi-layered structure of elements each somewhat similar to the element of Fig. 2. The multi-layered `structure includes a plurality of elongated units 36, each unit 36 being built up nom a plurality of superposed layers. Each unit 36 com- 6 prises a transparent conductive lstrip 38, an elongated strip-like portion of a layer `40 of electroluminescent phosphor, a composite layer made up of a row of electrically separate vconductive elements or squares 42 registered with the conductive strip 38 and an elongated striplike portion of a layer 44 of opaque insulating material surrounding the elements 42, a first photoconductivestrip 46, an electroluminescent phosphor strip 48, a second photoconductive strip 50, and an elongated conductor 52, such as narrow strip or line, laid down in that order. The superposed strips 48, 50 and S2 of each unit 36 are spaced from the next adjacent unit to form separate storage sections. The transparent conductive strip 38 and the row of conductive elements 42`are narrower than the strips 46, 48, and 50, and are mounted oit-center with respect thereto. The conductive line 52 is mounted off-center also but on the opposite side with respect to the conductive strip 38 and the row of conductive elements 42. The edge-to-edge spacing between the conductive line 52 and the elements 42 is approximately equal to the width of elements 42 and the transparent conductive strips 38. The conductive lines 52 are connected together and to one side of a voltage source 54. The transparent conductive strips 38 are connected together and to the other side of the voltage source 54. A light opaque shield in the form of a cup shaped member 55 may be mounted on the opaque layer 44 to lshield the photoconductive strips 46 and 50 from :ambient light.

The transparent conductive strips 38 may be formed by spraying tin chloride on the glass plate '34 through a mask while the plate is heated, or by evaporating gold, silver or aluminum, for example, through a mask in vacuum. The conductive elements 42 and conductive lines 52 may be formed by vacuum evaporation of metal or by silk screening of silver paste. The opaque insulating layer `44 may be formed of a black lacquer applied in the spaces between the elements 42. As an example of the physical dimensions, the transparent conductive strips 38 and the conductive elements 42 may be 10 mils wide and l mil thick. The strips 46, 48 `and 50 may each be 1 mil thick and slightly over 20 mil wide. The lateral spacing between the conductive lines 52 and the conductive squares 42 may be about l() mils.

The storage sections may be formed by stacking together the various layers as strips. Alternatively, continuous layers may be laid down over the entire layer 44 and elements 42, and then narrow grooves 56 may be cut through the layers, leaving the separate storage strips 46, 48 and 5). If desired, the narrow grooves may be filled with insulating material. Alternatively, continuous layers may be used with lines 52 symmetrically spaced Withrespect to conductive strips 38 so that two feedback elements are connected to each strip 52.

Considering an elemental area, each conductive element 42 and registering portions of the electroluminescent layer 40 and the transparent conductive :strip 38 constitutes an elemental indicating electroluminescent section or cell corresponding tothe cell 26 of Fig. l or 2. Also, the conductive element 42 and the conductive line 52 constitute two terminals of a storage device including the two photoconductive layers 46 and 50 and the second electroluminescent layer 48, and corresponding to the storage section 10a of Fig. 2. The storage section and the indicating section are electrically connected in series by the common conductive element or square 42. The opaque layer 44 is interposed between the two sections and cooperate with the elements 42, which are also opaque, to prevent light from reaching the'storage section from the indicating side.

In operation, a pulse of in put light L3 incident on lan elemental area of the second photoconductive layer l50 will trigger on `the storage section. The light pulse L3 will be amplified as light emitted by the elemental area of the second electroluminescent layer- 48 associated with -the photoconductive ".areaexcited.` The light emitted-bythe second electroluminescent layer 48 will feed back to the photoconductive layers and will not be visible. Visi- .ble -light L4 Will be emitted by the electroluminescent layer .40 which isV in series` with the storage section and the .voltage source. Because of the storage action, both electroluminescent layersrwill continue to emit light after the incident light pulse L3 is cut olf. Similarly, a light pulse L incident on a dilferent area will be visible as continuous light L6. Thus, an input light image will be stored inthe storage section, and the storage will be indicated by the indicating section.

The device of Figs. 3 and 4 is one which has a high degree of storage eiciency and one which is capable of recording light images of short duration and storing the same as visible picture information for as long a time as desired The storage sections and indicating sections are optically-separated so that the storage sections cannot be triggered on by the output light, thus eliminating crossrnodulation effects. The storage scctionscannot be triggered on by ambient light falling on the viewing side so that the device may vbe viewed in high levels of ambient light by shielding the storage section from ambient light. Because the storage and indicating sections are separate, 4the color of the output light may Ibe different than the color of the input and feedback light. For example, assuming the input image to be red light, the photoconductive layers of the storage section may be made of material having -a good response to this color light. An example of such a photoconductor is the cadmium sulfide powder referred toV above. The electroluminescent layer of the storage section may be made of an electroluminescent phosphor which emits light to which the photoconductor is very sensitive, such as a copper activated zinc sulfoselenide [Zn (S:Se): Cul, in the case of red light. Such a Vphosphor -is disclosed in copendng applications of Simon Larach and Robert M. Mazo, Serial Nos. 394,646 and 394,647, now U.S. Patent 2,847,386, iiled November 27, 1953. The electroluminescent ylayer in the indicating or'viewing sec-tion may be selected to give the desired color light output. If a blue light output'is desired, for example for photographing, the electroluminescent phosphor may be copper activated zinc sulfide.

Figs. 5V and 6 sho-w another form of light storage device 58 employing optically separate storage and indicating sections. The device 58 comprises a transparent insulating base 60, a transparent conductive coating 62, an electroluminescent phosphor layer 64, and an opaque insulat- -ing layer 66, arranged in that order to form an indicating section. VThe storage section comprises a perforated member 68 made of opaque insulating material on the opaque layer 66, the perfor'ations `69 being filled with superposed plugs of electroluminescent phosphor 70 and photoconductive material 72. A transparent conductive coating 74, such as silver paste, -is laid down in contact with the outer surface of the photoconductive plugs 72. The transparent conductive coatings or electrodes 62V and 74' are connected across a voltage source 76. A lightopaque shield in the form of a cup-shaped member 78 may be mounted on the outer surface of the conductive layer 74 to shield the storage section from ambient light.'

The perforated member 68 may be made of polystyrene dyed black. It may be joined to the opaque layer 66 by brushing one surface of the member 68 with an adhesive ora suitable solvent to render the surface tacky and then pressing the tacky surface -to the opaque layer 66. Alternatively, the opaque layer 66 and perforated member may be formed simultaneously by starting with an opaque plastic layer such as polystyrene mixed with lampblack. With the layer made soft, as. by the application of heat or the use of solvents, a suitable mold may be pressed into the layer to the desired depth'and then removed, leaving empty spaces'corresponding to the perforations 69. The perforations 69 may be round, as shown, or square, for example. If desired, a white lacquer may be used for the opaque 'layer 6,6 to provide a reflecting surface.

In operation, with no light incident on the photoconductive plugs 72, the impedance thereof is sufficiently high relative to the impedances of the electroluminescent plugs 7 0 and the electroluminescent layer 64 so that insuilicient current iiows through the series circuit to produce electroluminescence of either the electroluminescent plugs 70 or the electroluminescent layer 64. The magnitude of the voltage source 76 may be adjusted so that with given relative thicknesses of photoconductive and electroluminescent elements the deviceoperates below the threshold for visible electroluminescence.

When light L7 is incident on an elemental area, forexample an area defined by one of the photoconductive plugs 72, the conductivity of that-plug will increase, thus triggering' on the corresponding electroluminescent plug 70 and the elemental area of electroluminescent layer 64 in series therewith. The light emitted by the electroluminescent plug feeds back to re-excite the photoconductive plug and produce light storage. The storage action is made visible by light L8 emitted by the elemental area of electroluminescent layer 64. Y

The principal function of the opaque layer 66 is to isolate the storage section from the viewing section so that the photoconductive plugs can see neither the light emitted by the electroluminescent layer 64 northe ambient light incident on the glass plate 60 side of the device 58. This prevents the unwanted triggering from the viewing side of storage elements which normally should remain dark, and which correspond to the dark areas of an incident image. Unwantedtriggering of dark areas is prevented internally of `the storage section by the opaque member 68, which isolates each pair of storage elements, that is, superposed photoconductive and electroluminescent plugs 72 and 70 respectively, from adjacent plug pairs. If the layer 66 is made reecting, it reflects back to the photoconductive plugs all the light emitted by the associated electroluminescent plugs.

lIn some devices, adequate internal shielding of photoconductive plugs '72 may be achieved if only the photoconductive plugs are separated by the perforated member 68. If so, a continuous layer of electroluminescent material may be used in the storage section in place of separate electroluminescent plugs 70. From the standpoint of ease in manufacture, it may be desirable to extend the opaque perforated member 68 Vto the transparent conductive coating 62. In this case, the electroluminescent layer 64 may also be formed of plugs, and these plugs Vmay be separated from the electroluminescent plugs 70 by thin plugs of opaque material used in place of the opaque layer 66.

lf desired, the photoconductive plugs 72 and electroluminescent plugs 70 may be replaced by plugs each `made up of a homogeneous mixture of particles of photoconductive and electroluminescent materials, in which case the individual particles of the photoconductive and electroluminescent materials would be in light feedback relation and electrically connected together. Y

The invention thus provides a storage device capable of amplifying and storing light images with a high degree of storage efficiency, so as to respond to light or light images which are Weak or of short duration. The invention also provides a device in which cross-modulation is kept down to a minimum. Furthermore, the use of optically separated storage and indicating sections permits viewing of the output image in high levels of ambient light. It also permits the design of a storage device in which'the color of the stored output light `is different than the input or feedback light.

What is claimed is:

1l. An electroluminescent storage device comprising a first electroluminescent phosphor layer, an opaque layer on one surface of said phosphor layer, a transparent electrode on the other surface of said phosphor layer, a perforated member of opaque insulating material on said opaque layer, a'second electrolurninescent phosphor layer made up of phosphor material parti-ally lling the perforations of said opaque member and contacting said opaque layer, a photoconductive layer made up of photoconductive material superposed on said electroluminescent phosphor material and lling the remainder of 4said perforations, and a transparent conductive electrode in contact with said photcconductive layer.

2. An electroluminescent storage device comprising superposed layers including a first layer of electroluminescent phosphor material, a rst layer of photoconductive material, a second layer of electrolurninescent phosphor material, and a second layer of photoconductive material, in that order; a conductive element intermediate said -two layers of electroluminescent material and in contact with a surface of said first photoconductive layer, an elongated conductor on -the surface of said rst electroluminescent layer opposite said conductive element and registered therewith, and a second elongated conductor in contact with said second photoconductive layer and spaced laterally from said conductive element.

3. The invention according to claim 2, wherein an opaque insulating layer is interposed between said rst electrolum-inescent layer and said iirst photoconductive layer.

4. An electrolurninescent image storage device comprising a transparent insulating lbase, and a plurality of units on said base, each unit including: a first electroluminescent layer, an opaque linsulating layer, a first photoconductive layer, a second electroluminescent layer and a second photoconductive layer, in the order named; a row of conductive elements intermediate portions of said rst electroluminescent layer and said rst photoconductive layer; -a transparent conductive strip on the other side of said rst electroluminescent layer and registered with said row of conductive elements; and an 10 elongated conductor in contact with said second photoconductive layer and oset laterally from said row of conductive elements.

5. The invention according to claim 4, wherein the width of each of said transparent conductive strips and each of said rows of conductive elements is substantially equal to one-half the width of each of said layers.

6. The invention according to claim 4, wherein the width of each of said transparent conductive strips and each of said rows of conductive elements is substantially equal to the lateral spacing between each of said elongated conductors and each of said rows of conductive elements.

References Cited in the tile of this patent UNITED STATES PATENTS 2,594,740 De Forest et al Aug. 29, 1952 2,603,757 Sheldon July 15, 1952 2,721,808 Roberts et al., Oct. 25, 1955 2,805,360 McNaney Sept. 3, 1957 FOREIGN PATENTS 56,892 Denmark Oct. 23, 1939 157,101 Australia June 16, 1954 OTHER REFERENCES Mellon Institute of Industrial Research, Quarterly Report No. 3 of The Computor Components Fellowship No. 347, April 11, 1951 to July 11, 1951.

Journal Iof the Optical Society, vol. No. 44, No. 4, April 1954, Orthuber and Ullery, A Solid-State Image Intensifler, pages 297-299.

Loebner: Proceedings of the IRE, vol. 43, No. 12, December 1955, pp. 1897-1906. 

1. AN ELECTROLUMINESCENT STORAGE DEVICE COMPRISING FIRST ELECTROLUMINESCENT PHOSPHOR LAYER, AN OPAQUE LAYER ON ONE SURFACE OF SAID PHOSPHOR LAYER, A TRANSPARENT ELECTRODE ON THE OTHER SURFACE OF SAID PHOSPHOR LAYER, A PERFORATED MEMBER OF OPAQUE INSULATING MATERIAL ON SAID OPAQUE LAYER, A SECOND ELECTROLUMINESCENT PHOSPHOR LAYER MADE UP OF PHOSPHOR MATERIAL PARTIALLY FILLING THE PERFORATIONS OF SAID OPAQUE MEMBER AND CONTACTING SAID OPAQUE LAYER, A PHOTOCONDUCTIVE LAYER MADE UP OF PHOTOCONDUCTIVE MATERIAL SUPERPOSED ON SAID ELECTROLUMINESCENT PHOSPHOR MATERIAL AND FILLING THE REMAINDER OF SAID PERFORATIONS, AND A TRANSPARENT CONDUCTIVE ELECTRODE IN CONTACT WITH SAID PHOTOCONDUCTIVE LAYER. 